Method of evaluating flatness of a two-dimensional surface area

ABSTRACT

A method of evaluating flatness of a two-dimensional area of a surface of an unfinished workpiece prior to performance of a process step in which a prescribed operation is performed on the surface by a tool. A flatness value for the surface is calculated and used either to allow or to disallow the prescribed operation to proceed, and when allowed, uses the flatness value for parameter adjustment.

DOMESTIC PRIORITY CLAIM

This application claims priority of patent application Ser. No.15/013,338 filed Feb. 2, 2016, now U.S. Pat. No. 10,317,872 issued Jun.11, 2019, which claims priority of Provisional Patent Application No.62/202,231 filed Aug. 7, 2015. The entirety of patent application Ser.No. 15/013,338 is incorporated herein by reference.

TECHNICAL FIELD

This disclosure relates to a method of evaluating flatness of atwo-dimensional area of a surface of a workpiece before an operation isperformed on the surface during a manufacturing process.

BACKGROUND

An example of an operation in a mass-production manufacturing process isa plastic welding operation in which two plastic workpieces are joinedto each other to create a plastic assembly. Respective surface areas ofthe two workpieces are heated to melt, their respective surface areasand at least some of the plastic underlying each. The two workpieces arethen forced together at their melted areas and allowed to cool andsolidify into a joint which joins the two workpieces to each other.

Manufacturing variations in the mass-production manufacture of one, orboth, of the two workpieces can create dimensional differences fromworkpiece-to-workpiece in what are substantially dimensionally-identicalworkpieces. Such dimensional workpiece-to-workpiece differences can becompensated for if they are within specified dimensional tolerances.

One example of such variations in workpiece-to-workpiece is surfaceflatness which may be caused by the nature of the process which is usedto fabricate the workpieces. Plastic workpieces which are fabricated byblow molding processes are susceptible to varying degrees of surfaceflatness from workpiece-to-workpiece. Excessive variations in flatnessof one, or both, of two surfaces at which two workpieces are to bewelded together may impair a welding operation to such an extent that adefective joint is created and the assembly must be either re-worked orscrapped, reducing the efficiency of the manufacturing process.

SUMMARY OF THE DISCLOSURE

This disclosure relates to a method of evaluating flatness of atwo-dimensional area of a surface of a workpiece prior to performance ofa manufacturing step which performs a prescribed operation on thesurface by a tool.

The method comprises, a) at each of multiple locations within atwo-dimensional coordinate system for the surface which comprisesmutually perpendicular X- and Y-axes, measuring Z-axis coordinate datafor elevation of the surface along a Z-axis which is perpendicular toboth X- and Y-axes thereby defining X-axis, Y-axis, and Z-axiscoordinate data at each location; b) defining a best-fit plane whichbest fits to the combined X-axis, Y-axis, and Z-axis coordinate data atall locations; c) calculating a flatness value for the surface bysubtracting from the Z-axis coordinate data of the highest elevation ofall locations, Z-axis coordinate data of the defined best fit plane atthe location of the X-axis, Y-axis coordinate data of the locationhaving the highest elevation of all locations to yield a positivedifference, subtracting from Z-axis coordinate data of the defined bestfit plane at the location of the X-axis, Y-axis coordinate data of thelocation having the lowest elevation of all locations, the 7-axiscoordinate data of the lowest elevation of all locations to yield anegative difference, adding the absolute value of the negativedifference to the positive difference to calculate a sum representingthe flatness value of the surface; and d) comparing the calculatedflatness value of the surface with a maximum allowable flatness valuefor the surface.

Performance of the operation is allowed when the sum is not greater thanthe maximum allowable flatness value, but is disallowed when the sum isgreater than the maximum allowable flatness value.

The foregoing summary, accompanied by further detail of the disclosure,will be presented in the Detailed Description below with reference tothe following drawings that are part of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan diagram of a four-station work station havingrobots at three of the stations for performing repetitive operations onworkpieces.

FIGS. 2 and 2A collectively show a perspective view of a first of threestations which have robots.

FIG. 3 is a perspective view of the front of a processing unit which ispositioned by a robot at the first station.

FIG. 4 is a perspective view of the rear of the processing unit shown inFIG. 3.

FIGS. 5, 6, and 7 schematically show steps of a hot plate weldingprocess.

FIG. 8 is a perspective view of a hot plate assembly shown in FIG. 3.

FIG. 9 is a legend showing layers of a wall of a co-extruded blow-moldedtank for holding volatile liquid fuel.

FIG. 10 is a fragmentary perspective view of a master shape for afeature of interest in the wall of the tank of FIG. 2 including a crosssection through the feature of interest.

FIG. 11 is a view in the direction of arrow 11 in FIG. 10 and, includesthe hot plate assembly of FIG. 8 melting the feature of interest in thetank wall.

FIG. 12 is an enlarged fragmentary view in circle 12 of FIG. 11.

FIG. 13 is cross section view of the feature of interest after the hotplate assembly has been removed and a tube has been welded to thefeature of interest.

FIG. 14 is a three dimensional image display resulting from a scan of anarea of the tank containing the feature of interest.

FIG. 15 is a two dimensional image developed from the three dimensionalimage display.

FIG. 16 is another view of the two dimensional image showing a step inan analysis of the image.

FIG. 17 is another view of the two dimensional image showing anotherstep in the analysis.

FIGS. 18, 19, 20, 21, 22, and 23 illustrate a series of additional stepsin the analysis.

FIG. 24 is a perspective view showing diagrammatically an example of aresult of the analysis.

FIG. 25 is a perspective view showing diagrammatically how the result ofthe analysis is used to orient a hot plate to the feature of interest.

FIG. 26 is a perspective view showing diagrammatically how meltedmaterial may be displaced by the hot plate during a matching phase, orstage, of the melting process.

FIGS. 27, 28, and 29 illustrate a sequence of steps in measuring wallthickness of the tank at the feature of interest.

FIG. 30 is a perspective view of another type of heating element formelting plastic.

FIG. 31 is a side elevation view of another hot plate.

FIG. 32 is a perspective view in the general direction of arrows 32-32in FIG. 31.

FIGS. 33 and 34 are two more examples of steps like those of FIGS. 19and 20.

DETAILED DESCRIPTION

FIG. 1 shows a workstation 50 having a loading/unloading station 52 atwhich a part to be processed (i.e. a workpiece) is loaded into, andunloaded from, a fixture 54 on a turntable 56, a first processingstation 58 comprising a first robot 60, a second processing station 62comprising a second robot 64, and a third processing station 66comprising a third robot 68.

Four fixtures 54 are mounted on turntable 56 at 90° increments about acentral vertical axis 70 of the turntable, and stations 52, 58, 62, 66are arranged on a workplace floor 72 at 90° intervals about axis 70. Aprime mover indexes turntable 56 in precise 90° increments of rotationabout axis 70 to advance a fixture containing a workpiece which has beensecured at a fixed location on the fixture from one station to asucceeding station in the following manner.

After a workpiece has been secured in a fixture 54 at station 52,turntable 56 is indexed to advance the workpiece to station 58 forprocessing at the latter station while a workpiece which has beenprocessed at station 58 is concurrently advanced to station 62 forprocessing at the latter station, a workpiece which has been processedat station 62 is advanced to station 66 for processing at the latterstation, and a workpiece at station 66 is advanced to station 52 forunloading at the latter station. In this way each of the three stations58, 62, and 66 performs a specific operation on a workpiece whicharrives at the respective station before the workpiece is advanced to asucceeding station.

FIGS. 2, 2A, 3, and 4 show first robot 60 and a processing unit 74 whichis securely fastened to an arm 76 of first robot 60 for movement byfirst robot 60 within an orthogonal coordinate system by translation ofprocessing unit 74 along linear X, Y, and Z axes of first robot 60 andby rotation of processing unit 74 angularly in roll, pitch, and yawabout respective axes W, P, and R. First robot 60 is a commerciallymanufactured device known as a six-axis industrial robot having multiplemotors which are operated by a controller 79 for positioning any devicefastened to arm 76 within a coordinate system with six degrees offreedom (X, Y, Z, W, P, R). An example of such a robot is a Fanuc™125F.

FIG. 2 shows a fixture 54 into which a workpiece W has been loaded.Fixture 54 holds the workpiece secure such that the workpiece isstabilized against movement on the fixture although certain portions ofthe workpiece may be slightly deformed by an operation being performedat any station depending on the nature of the construction of theworkpiece and that of the operation being performed on it.

The particular example of workpiece shown in FIG. 2 is a plastic tank 77which has been manufactured by a co-extrusion process and which has afeature (feature of interest), common to all such tanks processed byworkstation 50, on which first processing station 58 performs aprescribed operation or operations.

Processing unit 74 comprises a three-dimensional vision system,including a three-dimensional scanning camera 78, for acquiring X, Y,and Z coordinate data defining the feature of interest on a tank 77which has arrived at first processing station 58, and a processor 80 forstoring and processing data. Processor 80 and controller 79 cancommunicate with each other via a data link 81. However, before firstprocessing station 58 is allowed to perform any operation on asuccession of what are substantially identical tanks, coordinate datawhich defines a master location for the feature of interest is developedby what may be called a teaching process for first robot 60. The visionsystem has what is known as a “smart” camera which embodies bothprocessor 80 and a scanner. A different vision system may have theprocessor located remotely from processing unit 74, such as in anelectrical cabinet on a workplace floor.

The teaching process can be best explained in the following way.

To begin, a tank which is to be used in the teaching process is loadedinto a fixture 54 and properly secured. The tank can be an exact mastermodel of a tank or an actual tank which has been measured to assure thatits relevant dimensions correspond sufficiently closely to the designintent represented by the master model that it can be consideredequivalent to the master model. First robot 60 is operated to acoordinate position within the six-axis coordinate system describedabove which has been determined to be an appropriate starting point forcamera 78 to begin a scan of a region of interest of the tank containingthe feature of interest on which an operation would be performed. Thatstarting point may be referred to as scan start position. First robot 60then moves processing unit 74 along a scan path at a constant speed in astraight line, which also moves camera 78 in the same way, while camera78 scans the region of interest. Processor 80 acquires data about theregion of interest as the scan proceeds and from that data, by a processwhich will be more fully described later, develops X,Y,Z,W,P,Rcoordinate data which defines the master location for the feature ofinterest in a coordinate system of processor 80 and records that data ina flash drive of processor 80.

Next, first robot 60 is operated by controller 79 to place a tool 82(see FIG. 2) of processing unit 74 in a home position whose coordinatesin the robot's coordinate system bear a specific positional relationshipto the feature of interest. Tool 82 is an actual tool (or an exactreplica) which would be used to perform a prescribed operation on thefeature of interest in a fixtured tank. Controller 79 has controls whichcan be manually operated by a person to place tool 82 in the tool's homeposition. The tool's home position may, for example, be a master homeposition from which tool 82 would begin to perform a prescribedoperation on the feature of interest in a fixtured tank.

With tool 82 in its master home position, its coordinates (X,Y,Z,W,P,R)are recorded in controller 79 to define the coordinates of the tool'smaster home position. Coordinates of the tool's master home position maybe identical to coordinates of a master home position for first robot 60if tool 82 is not movable on processing unit 74. The master homeposition for first robot 60 may be defined by the coordinates of the endof arm 76 to which processing unit 74 is affixed when first robot 60 isin its master home position.

When first robot 60 is positioned at its master home position, thecoordinates of tool 82 are different from the coordinates of the end ofarm 76 to which processing unit 74 is affixed because tool 82 is distantfrom the end of arm 76. When tool 82 is movable on processing unit 74,as will be eventually explained, the coordinates of tool 82, when at itsmaster home position are determined by modifying the coordinates of themaster home position of first robot 60 to take into account the positionof tool 82 on processing unit 74 relative to the end of arm 76 to whichprocessing unit 74 is affixed.

Next, tank 77 is removed from fixture 54 after which a second tank isloaded into the fixture and properly secured. The second tank is a partwhich has been manufactured by a mass-production process but ispresently unfinished. The nature of the particular manufacturing processmay inherently result in part-to-part variations, and if such variationsare ignored, they may affect accuracy of finishing operations asexplained above. First robot 60 is operated to position camera 78 at thesame scan start position which was used for the scan of the prior tank.Robot 60 once again moves processing unit 74 from scan start positionalong the same scan path and in the same manner as it did during thescan of the previous tank to scan the region of interest of the secondtank. Processor 80 acquires data about the region of interest as thescan proceeds and from that data develops X,Y,Z,W,P,R coordinate dataabout the feature of interest in the coordinate system of processor 80and records that data in the flash drive of processor 80.

Processor 80 then calculates differences between the X,Y,Z,W,P,Rcoordinate data for the feature of interest developed from the secondscan and the X,Y,Z,W,P,R coordinate data for the feature of interestdeveloped from the first scan.

Those differences are then transmitted from processor 80 to controller79 for use in controlling the robot's manipulation of processing unit 74and of any movement of tool 82 on processing unit 74 during performanceof the same prescribed operation on the second tank by applying thedifferences to re-locate the master home position to a modified homeposition from which tool 82 will begin to move when performing theprescribed operation on the second tank. For example, if the X-axiscoordinate data for the feature of interest developed from the secondscan is more positive than the X-axis coordinate data for the feature ofinterest developed from the first scan, the magnitude of the differenceis added to the X-axis coordinate of the master home position so thatany X-axis translation which is imparted to tool 82 during performanceof the prescribed operation on the feature of interest in the secondtank will begin at a modified home position which is offset from themaster home position by the calculated X-axis difference. In that way,the prescribed operation on the feature of interest in the second tankwill begin at the same position relative to that feature of interest asthe master home position is relative to the feature of interest in tank77. Similarly for such differences in the other five axes. If the X-axiscoordinate data for the feature of interest developed from the secondscan is less positive than the X-axis coordinate data for the feature ofinterest developed from the first scan, the magnitude of the differenceis subtracted from the X-axis coordinate of the master home position todefine the X-axis coordinate of the modified home position. Similarlyfor such differences in the other five axes.

Processor 80 compares data from the two scans by comparing the new X, Y,and Z data points which define the image of the feature of interest inthe second scan with the saved X, Y, and Z data points which define theimage of the feature of interest in the first scan. For each axis, thedifference between the data points from the two scans is calculated.Processor 80 then calculates roll, pitch, and yaw data (W, P, R) fromeach scan by trigonometric calculations and then calculates W, P, and Rdifferences between the scans, as will be more fully explained later.

Processing unit 74 as shown in FIGS. 3 and 4 comprises a frame 84 whichhas a central plate 86 shown disposed in a generally verticalorientation. Central plate 86 has a front face 88 and a rear face 90. Arobot mounting bracket 92 is disposed against rear face 90 and fastenedto central plate 86. Robot mounting bracket 92 has a circular end plate94 which is fastened to the distal end of robot arm 76 (robot's six axisarm).

Camera 78 and processor 80 are disposed vertically below and fastened tocentral plate 86 by a bracket 96 and posts 98 to dispose the camera andprocessor at an appropriate distance from the location where robot arm76 is fastened to end plate 94.

A track 100 is disposed against front face 88 and fastened to centralplate 86. A carriage 102 is retained on and guided by track 100 for backand forth travel in a straight line along the track. A pneumaticcylinder 104 has a cylinder body 106 mounted on central plate 86 by abracket 108 and a cylinder rod 110 which extends out of cylinder body106 into attachment with carriage 102 via a bracket 112 which isfastened to carriage 102. Cylinder rod 110 is displaced back and forthto move carriage 102 back and forth along track 100 as suggested byarrow 111.

A face of carriage 102 which is opposite track 100 contains two paralleltracks on which respective carriages 112, 114 are retained and guidedfor back and forth travel in straight lines along the respective tracksas suggested by arrows 116, 118. The tracks cannot be seen in the Figs.because their view is blocked by other parts of processing unit 74.

Pneumatic cylinders 120, 122 have respective cylinder bodies 124, 126mounted on central plate 86. A cylinder rod extends out of cylinder body124 into attachment with carriage 112, and a cylinder rod extends out ofcylinder body 126 into attachment with carriage 114. The cylinder rodscannot be seen in the Figs. because their view is blocked by other partsof processing unit 74. Each cylinder rod can be independently displacedback and forth to move the respective carriage 112, 114 independentlyback and forth along the respective track.

A valve mounting panel 128 containing various pneumatic control valvesis mounted via posts 130 on central plate 86. Connections from thevalves to the pneumatic cylinders are not shown. Those connections, aswell as other connections also not shown, such as electrical ones, areorganized to follow travel of each of the three carriages by guidancewhich is constrained by respective chain links 132, 134, 136. Each chainlink has a first end attached to frame 84 and a second end attached to arespective carriage. The connections enter at the first end and exitfrom the second end.

The coordinates of the position of tool 82 in the robot's coordinatesystem are a function not only of the coordinates of the position of theend of arm 76 but also a function of the position in which carriage 112is placed by pneumatic cylinders 104, 120. Assuming that the position ofarm 76 is defined by coordinates of the end of the arm, the location ofcarriage 112 on processing unit 74 is, as mentioned earlier, accountedfor when the coordinates of the tool are being calculated. Respectivesensors associated with the respective cylinders communicate data tocontroller 79 measuring the distance to which the respective cylinderrod is extended.

First processing station 58 performs a hot plate welding process inwhich tool 82 is a hot plate. A schematic example of hot plate weldingis portrayed in FIGS. 5, 6, and 7. The process comprises first andsecond phases, or stages, (FIG. 5) in which a hot plate 138 is used toheat ends of a first plastic part 140 and a second plastic part 142 atwhich the parts will be welded together. The surface temperature of hotplate 138 and the duration for which it is in contact with the parts'surfaces create sufficient melting of those surfaces to enable the partsto be welded together. In typical plastic welding processes, the hotplate temperature is within a range from about 30° C. to about 100° C.above the temperature at which the plastic will melt. A third stage ofthe process (FIG. 6) is a changeover phase in which, after an end ofeach part 140, 142 has been melted, the parts are moved out of contactwith hot plate 138. A fourth stage of the process (FIG. 7) is a fusionphase in which the ends of the parts which have been melted are firstplaced together under pressure to cause the melts to blend together andthen are held in that position long enough, at the same or a reducedpressure, for the melts to cool and solidify, thereby completing theweld.

The nature of certain processes for mass-producing plastic parts cancreate dimensional differences from part-to-part, two examples of whichare differences in wall thickness and differences in surface flatness.Such differences may be caused by variations in melt flow rate, humidityof raw materials, ambient temperature, etc. Other part-to-partdifferences involve a feature of interest on which an operation is to beperformed and they include the feature's shape and its location in therobot's six-axis coordinate system. As long as differences between anygiven part and the specified design intent, as represented by masterdimensions for the part, are within acceptable tolerances, that part isconsidered suitable for being welded to another part as long as thatother part itself is also within tolerance of its design intent. Yet ifthe melted portions of the within-tolerance parts are mismatched whenplaced together during a welding operation, such mismatch may adverselyaffect quality of the finished weld.

A surface of a plastic part which is to be melted by a hot plate shouldideally match the shape of a surface of the hot plate which will contactthe part surface, and the hot plate surface and the part surface shouldbe brought into full surface-to-surface contact at the exact locationspecified by the design intent. Failure to meet such requirements canadversely affect weld quality, and when plastic parts are being welded,part-to-part variations as described above can have such an effect onachieving consistent weld quality. Matching surfaces to be joinedinclude planar and non-planar surfaces.

The disclosed method can provide more consistent weld quality byperforming adjustments which compensate for part-to-part differencessuch as those described above.

When a plastic part is properly secured in a fixture and a hot platesurface is precisely positioned by a robot to the same coordinatelocation at which it is to make initial contact with a surface of thepart intended to be melted, part-to-part differences as described abovecan result in mismatch between the part surface to be melted and the hotplate surface. If mismatch occurs, force being applied by the hot plateto a part will initially displace some melt before eventually conformingmelted plastic to the hot plate geometry. Any melt which is displacedduring this conforming, or matching, stage is incorporated into flashbeyond the hot plate perimeter.

Processing unit 74 has a thermal imaging camera for taking a thermalimage of a surface area of a part after it has been heated for aspecified length of time to verify thermal distribution of a pool ofmaterial melted by a hot plate to assure that sufficient melting hasbeen achieved for the welding process to be completed. The image istaken after the hot plate has been moved away. If the image shows thatmelt is insufficient, the process is terminated to avoid the possibilityof making a bad weld.

After the melt has been conformed to the hot plate, the process proceedsas if full surface-to-surface contact had initially occurred. Forcebeing applied to the part by the hot plate can be maintained, or reducedto some minimum, while the hot plate continues to remain in contact withthe part during this heating phase.

Heat now penetrates into the part without any substantial displacementof melt. The temperature of the melt surface continues to rise slightlyand eventually reaches about 20° C. below the surface temperature of thehot plate.

At the same time as a feature of the part is being melted, a feature ofa second plastic part is being melted in a similar way, as explainedearlier.

At the end of the heating phase, the change-over phase occurs by movingeach part out of contact with the respective hot plate.

The fusion phase begins by pressing the parts together at their melts.The magnitude of the applied pressure depends on certain factors, suchas melt viscosity and part wall thickness. The pressure can bemaintained or reduced as the melts blend together and cool by heat flowfrom the melts both into the surrounding air and also more deeply intothe interiors of the parts. The weld is complete once the temperature ofthe solidified melts has dropped significantly below the crystallinemelting point or below the softening temperature. Quality of thefinished weld is affected by the severity of any mismatch occurring whenthe hot plate initially contacted the plastic. By compensating formismatch in any one or more of the axes of a six-axis coordinate systemas disclosed herein, significant improvement of weld quality ofmass-produced parts can be achieved.

The example of hot plate welding process which is portrayed by thedrawings described above is performed at first processing station 58where a circular flange 145 of a tube 143 (FIG. 13) is welded to a weldpad 157 (FIG. 2) of a tank 77.

Processing unit 74 comprises a first hot plate assembly 144 (FIG. 3)containing a first hot plate 146 and a second hot plate assembly 148containing a second hot plate 150. First hot plate assembly 144 isfastened to carriage 112 for movement on processing unit 74. First hotplate 146 is used to melt a portion of weld pad 157 and is an example ofa tool 82 whose position is controlled by controller 79 duringperformance of an operation on a workpiece. Second hot plate assembly148 is mounted on one leg of a right angle bracket 152 whose other legis fastened to central plate 86. Consequently second hot plate 150 isimmovable, and therefore has a fixed location, on processing unit 74.

First hot plate assembly 144 is shown by itself in FIG. 8 and comprisesa proximal end 156 via which it is securely fastened to carriage 112.First hot plate 146 is at a distal end of assembly 144. Electricconductors coming from an electrical panel, enter and pass through chainlink 134, and then pass out of chain link 134 to end at terminals 158which are fastened 180° apart to a circular side surface of a bandheater 155. The conductors provide an electric current path forelectricity to flow to and through band heater 155, heating first hotplate 146 in the process. Second hot plate 150 is heated in a similarmanner. Resistance temperature detectors (RTD's) 160 are fastened 180°apart to a circular side surface of first hot plate 146 and used tomonitor hot plate temperature and enable current to band heater 155 tobe controlled for proper temperature regulation. A first RTD is used foractual control while the second is used as a redundancy check on thefirst.

A gripping tool 154 is securely fastened to carriage 114 for movementwith that carriage on processing unit 74 toward and away from second hotplate 150. Gripping tool 154 functions to grip a tube 143 and move thetube to contact a flat bottom surface of flange 145 with second hotplate 150 for melting a portion of the flanges bottom surface. Oncesufficient melting has occurred, gripping tool 154 moves tube 143 offsecond hot plate 150.

Weld pad 157 is located within a region of interest in the wall of tank77 and comprises a raised formation having a circular annular topsurface 159 (FIG. 10) surrounding a circular hole 164 having a center165. As will be further explained below, a portion of the wallunderneath top surface 159 is melted by contacting surface 159 with acircular annular flat surface 161 of first hot plate 146 while at thesame time the bottom of tube flange 145 is being melted by second hotplate 150. After sufficient melting of both flange and weld pad,gripping tool 154 removes tube 143 from second hot plate 150 and placesthe melted bottom of flange 145 in contact with the melted top of weldpad 157 so that the melts can blend and cool to completion, after whichgripping tool 154 releases tube 143 and moves away.

Tank 77 is fabricated by blow molding of multiple plastic coextrusionsin accordance with known processes. The fabricated tank comprises amultiple layer wall which has a basic shape which comprises a bottomwall, a side wall, and a top wall. The specific shape of each of thesewalls is determined by the shape of the interior of the mold cavitywithin which the blow occurs.

Various operations, such as welding and boring for example, may beperformed on the blown tank to enable additional components to beassembled to the tank so that the tank can be used in the environmentfor which it is intended, such as a fuel canister or a fuel tank in amotor vehicle.

When a canister or tank is intended to hold a volatile liquid such asgasoline, the multiple layer coextrusion commonly includes one extrusionwhich in the blown canister or tank provides an EVOH layer 163 whichserves as a hydrocarbon barrier for preventing volatile gases fromescaping through the wall of the canister or tank.

FIG. 10 illustrates a portion of the tank wall at weld pad 157 and itssix coextruded layers. FIG. 9 describes those layers and shows therelative thickness of each layer as a percentage of the total wallthickness. Even though the EVOH layer 163 is closer to the interiorsurface of the tank wall than the exterior surface, the EVOH layer ispotentially at risk of being breached if it should be contacted by hotplate 146 during the melting phase of a welding operation due toexcessive mismatch of the hot plate to the wall during melting. Anexample of angular (W, P, R) mismatch is illustrated in FIGS. 11, 12,and 13.

FIG. 13 illustrates weld pad 157 and tube 1143 after having been weldedtogether as explained above. In this example, the orientation of flange145 is angularly tipped (i.e. not parallel) relative to the underlyingweld pad 157 to which it has been welded. The tipping is the result ofangular mismatch between weld pad surface 159 and surface 161 of firsthot plate 146 which caused a portion of the circumference of surface 161to penetrate the wall more deeply than elsewhere around thecircumference, and as a consequence breach a portion of EVOH layer 163.Although a melted portion of tube flange 145 is placed onto a meltedportion of weld pad 157, the finished weld may not seal the breach andconsequently be defective.

FIG. 12 illustrates an example of how such angular mismatch canpotentially damage EVOH layer 163 if hot plate surface 161 comes intocontact with the EVOH layer. Locational (X, Y, Z coordinate) mismatch isnot present in this example, but if present, could potentially damageweld pad 157 if hot plate 146 were to make inaccurate contact withsurface 159.

Due to part-to-part variations, the X, Y, Z coordinates of a feature ofinterest, such as weld pad 157, on which a tool, such as first hot plate146, is to perform one or more operations, can vary as much as about ±½″when the workpiece is placed in a fixture associated with a machine thatcontrols the operation of the tool. Also angular mismatch between afeature of interest and a tool in W, P, and R coordinates can vary up toabout ±3°.

Robot 60 and processing unit 74, as described above, are capable ofcompensating for such mismatches and thereby enable a repetitiveoperation of a mass-production process to be performed with moreconsistent accuracy from part to part. Such a capability provides higherquality finished parts and can significantly reduce, or even eliminate,production of non-compliant parts.

The manner of compensating for both locational (X, Y, and coordinate)mismatch and angular (W, P, R coordinate) mismatch has been explainedearlier in a general way. Further detail will now be presented withreference to the example involving tank 77 and tube 143.

Robot 60 operates to position processing unit 74 at a starting locationand then move processing unit 74 along a defined path while camera 78scans an area which should contain the region of interest. During ascan, robot 60 moves processing unit 74 along the same path but becauseof part-to-part variations, the coordinates of the feature of interestcan vary. The completed scan contains coordinate data for the region ofinterest which can be visually portrayed on a two-dimensional screen asa three-dimensional image which can be manipulated for viewing fromdifferent directions. FIG. 14 shows a two-dimensional image of weld pad157 as viewed from one direction, but the actual scan data from whichthe image was developed contains X, Y, and Z coordinate data. FIG. 14 ispresented to facilitate the reader's understanding of how processor 80functions, and the fact that the scan data can be portrayed as it is inFIG. 14 should not be construed to imply that such an image is actuallycreated as a matter of necessity. It is from the X, Y, Z scan data thatW,P,R coordinate data is obtained as mentioned earlier and as will bemore fully explained later.

Processor 80 contains stored master data for any one or more geometricaspects which uniquely define the feature of interest. In the presentexample, those geometric aspects of weld pad 157 are annular surface 159and hole 164 (an annulus surrounding a hole). For each tank 77,processor 80 uses the master data for those geometric aspects toidentify the feature of interest in the scan data. Once the feature ofinterest has been identified, processor 80 derives a two-dimensional X,Y image (FIG. 15) from the X, Y, Z scan data. As suggested by FIG. 2,the path of the scan is generally parallel with annular surface 159.Consequently, the X, Y scan data lie in a plane which is parallel withthe scan path while Z scan data for each X, Y data point isperpendicular to that plane. Z coordinate data can be obtained by use ofthe gray scale method.

Using the X, Y master data for surface 159 and hole 164, processor 80analyzes the X, Y scan data to locate surface 159 and hole 164. If theyare not located, they are considered too far out of tolerance forsurface 159 to be melted. In the present example, the analysis takesplace within a defined area circumscribed by an imaginary circle 167shown in FIG. 16. Imaginary circle 167 is defined by data in processor80 and has a diameter greater than that of hole 164 and can be greaterthan an outer perimeter 169 of surface 159 depending on the partgeometry and feature of interest. If the analysis discloses that surface159 and hole 164 lie within the defined area, processor 80 uses X, Yscan data to calculate the hole's cross sectional area for comparisonwith the master data to determine if the hole size is within toleranceand to determine X, Y coordinate data for center 165 of hole 164.

Processor 80 also contains X, Y data defining two concentric imaginarycircles 173, 175 (FIG. 17) whose pre-set diameters bound an annulus 177representing an area of surface 159 which is to be melted. The center ofannulus 177 is centered on a center 166. Processor 80 places center 166on X,Y coordinate data for center 165 of hole 164 to cause annulus 177to overlie a portion of surface 159, thereby defining the annular zoneof surface 159 within which Z coordinate data is then obtained by thegray scale method mentioned above.

FIG. 18 shows a curved line 179 representing Z-axis coordinate dataobtained for surface 159 at each of a number of X, Y locations aroundthe defined annular zone. The example in FIG. 19 uses twenty-eight X, Ylocations at which Z-axis data is obtained, but a larger number oflocations may be used to increase Z-axis resolution. Using the Z-axiscoordinate data at those locations, processor 80 calculates a “best fit”plane 181 for fitting to that data. Some portions of curved line 179 areabove plane 181 while other portions are below plane 181, and whileplane 181 is shown horizontal in FIG. 18, whether it is or is nothorizontal depends on the geometry of surface 159 as determined by theZ-axis data which define it. In any event, X, Y, Z coordinates of anypoint on plane 181 are established and the Z coordinate value can becalculated from the Z coordinate value for the same X,Y coordinate oncurved line 179 by algebraically adding the Z-axis distance betweenplane 181 and curved line 179 which is negative at an X,Y location whereline 179 is below plane 181 and positive at an X,Y location where line179 is above plane 181.

Now that plane 181 has been defined in X, Y, Z coordinates, W, P, Rcoordinates can be calculated. How this is done is described withreference to FIGS. 21-23.

Three points on plane 181 are selected. FIG. 21 shows a first point 185at the center of the crosshairs. The X, Y, Z coordinates of that pointappear in the accompanying chart 187 to the left.

FIG. 22 shows a second point 189 spaced along the Y axis a distance (25mm in the example) from first point 185. The X, Y, Z coordinates ofsecond point 189 appear in the accompanying chart 191 to the left. Thedifference between the Z-axis coordinate of second point 189 and that offirst point 185 is calculated. Using principles of trigonometry, thetangent of the ratio of that difference to 25 mm is calculated and thattangent defines a pitch angle.

FIG. 23 shows a third point 193 spaced along the X axis a distance (25mm in the example) from first point 185. The X, Y, Z coordinates ofthird point 19:3 appear in the accompanying chart 195 to the left. Thedifference between the Z-axis coordinate of third point 193 and that offirst point 185 is calculated. Using principles of trigonometry, thetangent of the ratio of that difference to 25 mm is calculated and thattangent defines a roll angle.

FIG. 24 shows the best-fit plane 181 passing through weld pad 157. Thetwo zones 199, 201 are portions of surface 159 which overlie plane 181.FIG. 25 shows two zones 203, 205 which are below surface 159 and whichwill have to be melted in order to enable a good weld to be achieved.Hot plate surface 161 is positioned by robot 60 to be parallel tobest-fit plane 181 as the hot plate moves toward weld pad 157. Duringthe matching phase, some melted plastic is displaced until surface 159becomes fully matched to the underlying portion of the weld pad which isto be heated. This is portrayed in a general way in FIG. 26 wheredisplaced plastic is indicated by the numeral 207. Once surface 161 isfully matched to the plastic, the hot plate need not be advancedfurther.

If annulus 177 were larger, making surface 159 a smaller zone of theannulus, Z-axis coordinate data could be obtained for annular surfacesat each of a number of X, Y locations around additional annular surfacezones concentric with surface 159 to develop more topology of thesurface of annulus 177.

Surface flatness is a factor which deserves consideration in a weldingprocess and should be taken into account especially when a weld is toprovide a hermetic seal. If its flatness is excessive, a weld surfacemay be distorted enough to look like a “potato chip”. Such a surface maybe considered to have hills and valleys with various positive andnegative elevations relative to a best-fit plane. Curved line 179 andbest-fit plane 181 are used to calculate a flatness value for surface159 in a way analogous to running a depth gauge along the surface andtaking depth measurements at a number of locations, as suggested byFIGS. 19 and 20. Instead of using a gauge however, the distance betweenplane 181 and a point on curved line 179 which is farthest above plane181 and the distance between a point on curved line 179 which isfarthest below plane 181 is calculated by processor 80 (FIG. 19). Thesum of those distances is defined as the flatness value for surface 159(FIG. 20).

A standard (i.e. nominal) pre-set time for the matching phase and astandard force for the hot plate to apply to the plastic during thematching phase in order to completely melt a weld pad 157 for fullconformance with the surface of the heating tool may be insufficient toconform the portion to be melted to the hot plate geometry if theflatness value for the weld pad is too great. While a flatness valuesuch as 0.2 mm may be suitable for standard pre-set melt times, aflatness value can often vary between 0.5 and 1.0 mm due to moldingwarpage.

If parameters such as time, temperature and pressure are set for allwelds based a nominal 0.5 mm flatness value, surfaces having, flatnessvalues close to a 0.5 mm may be properly melted. However, surfaceshaving significantly smaller flatness values are apt to be melted toomuch, unnecessarily displacing material. On the other hand, surfaceshaving significantly greater flatness values would be insufficientlymelted. Processor 80 allows the hot plate to perform the prescribedoperation on a workpiece when the flatness value lies within a flatnesstolerance range and disallows the hot plate from performing theprescribed operation when the flatness value does not lie within theflatness tolerance range. When processor 80 allows the prescribedoperation to be performed on workpieces, the flatness value for eachsurface 159 is used by robot controller 79 to set at least one parameterfor the prescribed operation, such as controlling the cycle time of thematching/heating phases so that proper melting occurs.

For example, depending on the measured flatness value, for each 0.1 mmvalue that the flatness varies, another 2 seconds of time can be addedor removed (if we consider 0.5 mm as a nominal value) to overcome this.If thirty seconds is required to melt a weld pad having a 0.5 mmflatness value, a weld pad having a 0.7 mm flatness value, the timeparameter would be thirty-four seconds. This would provide nominalmaterial displacement and complete surface melt as shown in FIG. 26where plastic which has been displaced as flash during the matchingphase is indicated at 207 and conformed melting at 209. Smaller flatnessvalue provides for shorter cycle times since the material does not needto be displaced as much in order to cover the entire weld pad area.Parts which are more out of flatness require more matching time in orderto displace all of the material and then heat the unmelted materialunderlying the melt once the matching phase has finished.

To summarize, hot plate welding comprises a succession of four stages.

Matching is a first stage in which a hot plate is controlled by a loadcell to provide force sufficient to conform a surface of a workpiece,such as a fuel tank, to the hot plate geometry, and in doing soeliminate surface irregularities and create a flat underlying surfaceready for a heating stage. Material displaced during the matching stageis incorporated in the flash past the hot plate perimeter. Time for thisstage is determined experimentally usually by trial and error until thedesired result is achieved.

Heating is a second stage which starts immediately after the matchingstage without any mechanical movement of the hot plate. The force of thehot plate used during the matching phase may be decreased by controllingthe load cell so that the plastic underlying the hot plate surface meltswithout any displacement of melted plastic, with heat energy beingtransferred from the hot plate via conduction. Heating time may bedetermined theoretically or experimentally and checked through themicrotome process until a specified heat affected zone depth (0.4 mm forexample) is achieved.

Change-Over is a third stage. The hot plate is removed from theworkpiece, such as from a melted fuel tank surface in the illustratedexample. This is followed by positioning a second workpiece, having amelted surface, directly over the melted surface of the first workpieceand moving the workpieces together to place the melted surfaces togetheras a pool of melt. Change-Over time is controlled to avoid excessivesurface cooling of the melted surfaces before they pool together.

Fusion is a fourth stage of the process in which force is applied to thepooled plastic melts and they are allowed to cool and solidify into afinished joint. Force is applied and controlled by the load cell tosqueeze a desired amount of melt into flash around the joint. Having toolow a force may not allow for entrapped air to be removed. Trapped aircauses less intimate contact between the workpieces at the weldinterface. Having too high a force may squeeze an excess of melt out ofthe joining area, which can lead to creation of what is commonly calleda “cold weld” (virgin un-melted materials below the joining area act asa stop resulting in a weak weld).

Flatness value cannot be the sole basis for determining the timeparameter for the matching stage. FIG. 33 and FIG. 34 are flatness valuediagrams, illustrating two more examples which have identical flatnessvalues, but quite different shapes because of different volumes ofplastic which must be melted during the matching stage before theheating stage can commence. Therefore, assuming that the same hot platetemperature is used for the matching stage in each example, the matchingtime parameters for material displacement will be quite different.

Based on the area under the curve in each example, the temperature ofthe hot plate, and the force used for the matching stage, the time valuecan be determined. Once the time parameter for the matching process isexpired, the heating stage can start.

The area under each curve can be calculated by software which processesthe data points of each curve. Using a nominal value for the forceparameter and a nominal value for the hot plate temperature parameter,and using the area under each curve as a variable, the time parameterfor the matching stage of each example is calculated using amathematical model. If any of the two parameters and the area variablechanges, the time parameter is corrected by a mathematical model.

FIG. 33 shows a surface which is basically flat except for a hill H.FIG. 34 shows a surface which is basically flat except for two valleys Vwhich are separated a flat surface. Comparison of the two Figures showsthat, for the same hot plate temperature, the surface of FIG. 33requires less time for the matching stage because only plastic of hill Hneeds to be melted in order to complete the matching stage whereas thesurface of FIG. 34 requires more time because all the material above thebottoms of the valleys needs to be melted.

For maximizing the efficiency of a process, a time constraint may beimposed on cycle time between the beginning of the matching stage andthe end of the fusion stage. When a hot plate is used as the heatsource, varying its temperature is relatively slow, making force appliedby the load cell a controlling factor in achieving compliance with thecycle time constraint. When an IR heater is used as the heat source, itstemperature can be more quickly changed, and that makes both it andforce applied by the load cell controlling factors in achievingcompliance with the cycle time constraint.

Thickness of a weld pad wall is another variable inherent in a blowmolding process. Measuring wall thickness allows the position of theEVOH layer to be determined. This can be done by a measuring devicemounted on processing unit 74. After processing unit 74 has completed ascan of a weld pad, wall thickness at several locations around the weldpad is measured. Processor 80 is operable to allow the hot plate toperform the prescribed operation on a workpiece when the measures ofthickness lie within a thickness tolerance range and to disallow theprescribed operation from being performed when a measure does not liewithin the thickness tolerance range.

The EVOH layer represents approximately 3% of the total wall thicknessand if manufactured correctly, it is located at 70% of the wall belowsurface 159. Hence, in a wall having a 7.0 mm thickness, the EVOH layershould be at a depth of 4.9 mm from surface 159.

The ability to measure wall thickness and locate the EVOH layer enablesaccurate calculation of the quantity of material required to bedisplaced during the matching phase (calculated from flatness value) andthe quantity of material in the weld pad to be melted so that the EVOHlayer is not breached.

If wall thickness is measured at three locations as: 7.0 mm, 6.5 mm, &6.0 mm (6.0 mm being a minimum value for a worst case scenario), theEVOH layer depth should be 4.2 mm. If the flatness value is 0.5 mm, thedepth would be 3.7 mm due to material displacement. This leaves 5.5 mm(6.0 mm-0.5 mm) of the wall thickness available for melting withoutbreaching the EVOH layer. If the heating phase melts material to a depthof 2.0 mm, 1.7 mm of material above the EVOH layer is not melted and theEVOH layer is not breached.

FIGS. 27, 28, and 29 show one way to measure wall thickness. A lasersensor 211 is mounted on a bracket 213 which is itself mounted onprocessing unit 74. Robot 60 positions bracket 213 such that a topsurface of a foot 215 of the bracket is placed against the interiorsurface of the weld pad wall as in FIG. 29. Sensor 211 emits a beam(FIG. 28) which provides a measurement of the distance 217 from thesensor to surface 159 of weld pad 157. The distance 219 from the sensorto the top surface of foot 215 is known. The difference 221 betweendistances 219 and 217 is the thickness of the tank wall at the locationof the weld pad.

Another component which can be used for plastic welding is a lasertemperature probe which measures temperature of a component which is tobe heated and uses the measurement to automatically re-adjustparameters. Cold components have their own set of parameters which areused for processing and will need longer heat cycle time than warmercomponents. Temperature of warmer components allows for the heat cycletime to be decreased. For example, if a cold component requires 30seconds of heat cycle time, a warmer component at 70° C. might requireonly 25 seconds of heat cycle time because residual heat is alreadypresent in the component. Algorithm calculations can provide timeparameter based on the temperature measured.

Short wave and medium wave infrared (IR) is another welding method whichis similar to hot plate welding. The surfaces of the components to bejoined are not placed in contact with a hot plate but rather are heatedby direct IR exposure for a sufficient length of time to melt portionsof the components which are to be joined. Once the surfaces have beensufficiently melted, the IR source is withdrawn from the components,they are then placed together at their melts, and the melts are allowedto solidify. Whether standard or custom IR heating elements are used inorder to conform to part geometry, distance at which an element isspaced from a component is controlled. An example of an IR bulb 229 isshown in FIG. 30.

Standard industry practices do not allow for more than 0.5 mm deviationfrom a set nominal distance to a surface being heated. The time andpower parameters are changed if the deviation is greater. Because IRwelding is a non-contact method of plastic joining, flatness valuescalculated in the manner described above can be used to change the timeparameter. Larger flatness values will increase the heating time, whilesmaller ones will decrease the heating time. The process described aboveenables an IR element to be properly oriented to the surface to beheated and to be placed at an appropriate distance from that surface.

The process described above can be applied to heating a non-planarsurface. FIGS. 31 and 32 show a hot plate 230 which, unlike the flatplanar surface 161 of hot plate 146, has a non-planar heating surface232 which surrounds a cavity 234. Surface 232 is electrically heated inthe same way as surface 161 of hot plate 146 by the use of standardindustry heaters such as flexible heaters, band heaters or cartridgeheaters.

A component (not shown) has a surface be heated whose shape correspondsto surface 232. The component's surface whose shape corresponds tosurface 232 is the feature of interest and can be uniquely identified byits shape at certain locations around that surface.

The system which has been described can be used in multiple robotworkstations, such as in FIG. 1, where the positional differencescalculated at the first workstation are transmitted to subsequent robotsat workstations to which a fixtured workpiece is advanced for additionaloperations. By advancing a fixtured workpiece from the first workstationto a second workstation with sufficient accuracy to place the fixture atthe same location relative to the robot at the second workstation as thefixture was to the robot at the first workstation, the controller at thesecond workstation can use those differences to modify the master homeposition and create a modified home position for its tool. Consequently,the second workstation need not have a scanning camera nor perform ascanning process on a workpiece. Other process parameters such assurface flatness and wall thickness can also be transmitted withadjustments being made in real time.

While the embodiment which has been illustrated and described performsan operation on a workpiece by moving the tool relative to a stationaryworkpiece, principles disclosed herein may be applied to an embodimentin which the tool is stationary and the fixtured workpiece is movablerelative to the stationary tool. In the embodiment which has beenillustrated and described, camera 80 is movable with, but not movableon, processing unit 74. However camera 80 could be movable on processingunit 74 in the same way as tool 82 is movable on processing unit 74. Inthat case, coordinates of the position of camera 80 in the robot'scoordinate system would be a function not only of the coordinates of theposition of the end of arm 76 but also a function of the position ofcamera 80 on processing unit 74. The ability to move camera 80 onprocessing unit 74 would allow a scan of a feature of interest on a partto be made by movement of the camera alone while the end of robot arm 76remains stationary.

The principles disclosed herein are adaptable to processes forperforming operations other than hot plate melting of plastic. Examplesof other operations include, but are not limited to, assembly, drilling,and cutting operations.

What is claimed is:
 1. A method of evaluating flatness of atwo-dimensional area of a surface of an unfinished workpiece prior toperformance of a process step in which a prescribed manufacturingoperation is performed on the surface by a tool, the method comprising:a) at each of multiple locations within a two-dimensional coordinatesystem for the surface which comprises mutually perpendicular X- andY-axes, measuring Z-axis coordinate data for elevation of the surfacealong a Z-axis which is perpendicular to both X- and Y-axes therebydefining X-axis, Y-axis, and Z-axis coordinate data of the surface ateach location; b) defining a best-fit plane which best fits to combinedX-axis, Y-axis, and Z-axis coordinate data of the surface at alllocations; c) calculating a flatness value for the surface bysubtracting from the Z-axis coordinate data of the highest elevation ofthe surface at all locations, Z-axis coordinate data of the defined bestfit plane at the location of the X-axis, Y-axis coordinate data of thelocation having the highest elevation of the surface at all locations toyield a positive difference, subtracting from Z-axis coordinate data ofthe defined best fit plane at the location of the X-axis, Y-axiscoordinate data of the location having the lowest elevation of thesurface at all locations, the Z-axis coordinate data of the lowestelevation of all locations to yield a negative difference, adding theabsolute value of the negative difference to the positive difference tocalculate a sum representing the flatness value of the surface; d)comparing the calculated flatness value of the surface with a maximumallowable flatness value for the surface; and e) when comparison of thecalculated flatness value of the surface with the maximum allowableflatness value discloses that the calculated flatness value is notgreater than the maximum allowable flatness value, a tool performs aprescribed manufacturing operation on the surface of the unfinishedworkpiece.
 2. The method set forth in claim 1 in which the tool performsthe prescribed manufacturing operation on the surface of the unfinishedworkpiece by melting material in the surface of the workpiece.
 3. Themethod set forth in claim 2 further comprising using Z-axis coordinatedata for at least some of the locations to calculate an estimate of avolume of plastic which is to be melted during a matching phase, andusing the calculated estimate to set one or more operating parametersfor the tool to enable the tool to perform the melting in accordancewith the estimate.
 4. The method set forth in claim 1 in which, when theflatness value is not greater than the maximum allowable flatness value,using the flatness value to set at least one parameter for theprescribed manufacturing operation which the tool is to perform on theunfinished workpiece.
 5. The method set forth in claim 4 in which, whenthe flatness value is not greater than the maximum allowable flatnessvalue, using the flatness value to set length of time for which the toolperforms the prescribed manufacturing operation on the unfinishedworkpiece.
 6. The method set forth in claim 5 further comprisingmeasuring thickness of a wall of the unfinished workpiece which containsat least a portion of the surface, and causing the tool to perform theprescribed manufacturing operation on the unfinished workpiece when themeasured thickness lies within a thickness tolerance range anddisallowing the tool from performing the prescribed manufacturingoperation on the unfinished workpiece when the measured thickness doesnot lie within the thickness tolerance range.
 7. The method set forth inclaim 2 further comprising taking a thermal image of material which hasbeen melted by the tool and verifying thermal distribution of a pool ofmaterial melted by the tool.
 8. The method set forth in claim 7 furthercomprising measuring temperature of the unfinished workpiece before thetool begins to melt material in the surface of the unfinished workpieceand controlling length of time for which the tool melt material in thesurface of the unfinished workpiece as a function of measuredtemperature.
 9. The method set forth in claim 3 in which using thecalculated estimate to set one or more operating parameters furthercomprises using the calculated estimate to set one or more oftemperature of the tool and force applied by the tool to the plastic.